Dual field optical system



Fb. 2, 1960 R, H, TRlMBLE 2,923,202

vDUAL. FIELD OPTICAL SYSTEM Original Filed Aug. 21, 1948 2 Sheets-Sheet 1 ArraqNsY Feb- 2 1960 R. H. TRIMBLE 2,923,202

DUAL FIELD OPTICAL SYSTEM Original Filed Aug. 21, 1948 2 Sheets-Sheet 2 Eig. 2

A TTOQA/E V United States Patent O DUAL FIELD OPTICAL SYSTEM Richard H. Trimble, Los Angeles, Calif., assignor to Northrop Corporation, a corporation of California Original application August 21, 1948, Serial No. 45,548. Divided and this application May 26, 1953, Serial No. 357,553

Claims. (Cl. 88-57) My invention relates to optical systems, and, more particularly, to optical systems having both a narrow angle field and a wide angle field.

In star tracking systems utilized, for example, in conjunction with the automatic celestial navigation of vehicles such as aircraft, a telescope is used to focus the light from a star into an image deviation detector such as a scanner and a photocell, in order that the detector be able to detect a slight movement of the image away from a control point. The detector output is then used to initiate operation of a servo system attached to the telescope, to return the tracking axis to star sight line alinement thereby centering the image again on the contro point. l

The movement of the star image for a given angular movement of the telescope is greater, the greater the focal length of the telescope. Thus, tracking accuracy is increased with an increase in telescope focal length. Unfortunately, an increase in focal length decreases the width of the angular field over which the telescope and image detector can pick up a star to be tracked.

These antagonistic conditions might normally lead to a compromise between accuracy and width of image field. However, automatic star tracking in celestial navigation requires both accuracy and a wide star seeking field. It is an object of the present invention to provide a means and method of simultaneously obtaining accurate tracking and a wide angle image field, in order that relatively large star limage deviations from a control point will not cause the star image to be lost by the tracking system.

In broad terms, the invention comprises the use of an optical system providing a primary field close to the control point where high accuracy of tracking is obtained, and also providing a wide angle secondary eld in which the star image is prsent, even when there is far too much angular deviation of the tracking axis from the sight line to the star to maintain the star image in the primary field. The present application is. a division of my prior application Serial No. 45,548, filed August 21, 1948.

The following definitions will be of value in understanding the ensuing description of the invention:

Telescope.-An optical system capable of projecting an image of a star in a focal plane.

Tracking axis-The optical axis of the telescope.

Control point-Point where the star image lies in the optical axis of the telescope at or near the focal plane of the optical system.

Sight [ina- Straight line from star to control point.

Scannen-Rotating shutter, or equivalent, positioned in the path of the light beam at or near the image focal plane which, together with a cooperating light responsive electrical device feeding into a phase detector, constitutes an image detection system sensing and indicating image deviation direction from the control point.

Image field-The entire field at or near the focal plane wherein the position of the star image can be sensed by the image detection system.

Primary image.-A star having maximum response over the image field to angular movements of the tracking axis with respect to the sight line.

Secondary image- A star image having minimum response over the image field to angular movements of the tracking axis with respect to the sight line.

Primary field An area of the image field including a portion immediately around the control point where maximum sensitivity and accuracy of image deviation detection is desired and over which the primary image moves.

Secondary field An area around the control point where the secondary image remains in the image field after displacement of the tracking axis from the sight line by an angle sufficient to move the primary image entirely out of the image field, or, to completely remove the primary image.

Two types of optical systems will be described herein as being capable of practicing the invention.

IIn one preferred system a cassegranian telescope, normally utilizing two mirrors, has a secondary mirror replaced by a combination lens and mirror which forms two star images in the image field. One of these images, the primary image, is formed by light reflected to the rear surface of the mirror-lens by the primary mirror, this light then being reflected to the image field by this rear surface, as in a conventional cassegranian system. To obtain this function, the rear surface of the mirror-lens is partially silvered. This latter optical path is of long focal length and therefore provides high tracking accuracy in the image field as long as this primary image remains therein. The other image, the secondary image, is formed by light from the star passing through the mirror-lens alone, thereby providing a short focal length in the optical system and for relatively large angular deviations of the tracking axis from the sight line during which the secondary image remains in the image field. When the primary image leaves the field, tracking is continued on the secondary image which still remains in the image field. Thus, both high tracking accuracy and a wide angle tracking field is attained.

A second preferred means by which the same results can be obtained is by the use of a cassegranian telescope utilizing conventional primary and secondary mirrors and in which a non-linear lens, such as a toroidal lens, is placed between the secondary mirror and the scanner. This lens is composed of two portions, a central portion which is linear and which has no lens on the incoming light beam, so that the primary image in the primary image field near the control point has maximum response to angular movements of the telescope, and an outer toroidal portion which can form a substantially radially stationary secondary star image in the secondary field around the primary image field, over wide angular deviations of the tracking axis from the sight line. After the primary image has left the primary field, the secondary image appears in the secondary field. This latter image is then used to bring the star image back into the primary field where high tracking accuracy is possible. This can be done by the scanner and photocell response even though the secondary image is radially stationary, because the circumferential position of the secondary image will determine the direction of the centering movement of the telescope, and this centering movement will continue until the secondary image disappears from the secondary field and appears as the primary image in the primary field, and until this primary image is centered on the control point.

Thus, the only differences in the two systems are that in the mirror-lens system the primary and secondary fields are co-extensive, and two images are seen when image deviations from the control point take place. Both of these images move radially across the image field, the

primary image faster, the secondary image slower, as angular deviation takes place.

In the toroidal system the secondary image, when in the secondary field, is substantially radially stationary under the same circumstances. In both systems, however, the circumferential position of the secondary image while in the secondary field indicates the direction the tracking axis has to be moved to register the tracking axis accurately with the sight line to place the primary image on the control point.

The end result in both cases is identical. When the telescope is first aimed in the direction of the star to be tracked, a wide field is necessary to bring the star into the image field especially if this aiming is done automatically as by a predetermined angular setting. Accurate tracking is then possible when the star image is brought into the primary field. Such an arrangement is also highly desirable in star tracking from moving vehicles subject to various unexpected accelerations, as once the star image has been lost by the scanner, it is highly improbable that it ever would be regained, and all automatic control of the vehicle over a predetermined course would be completely lost.

Other objects and advantages will be more clearly understood by reference to the drawings, in which:

Figure l is a diagram of a star tracking system.

Figure 2 is an'optical diagram showing the creation of a primary and secondary field by a mirror-lens used in a cassegranian reflecting telescope.

Figure 3 is an optical diagram showing the creation of a primary and secondary field by a toroidal lens used in a cassegranian reflecting telescope.

As shown in Figure 1, a telescope 1 is mounted on a platform 2 in a moving vessel for example, and is pointed in the direction of the light to be sought, such as light from a star S, to make the tracking axis A of the telescope generally coincident with the sight line to the star. The optical system (later described) of the telescope focuses an image of the star onto a light splitter 3 which in the embodiment illustrated reflects approximately half of the incident light at substantially a right angle to the tracking axis. The light rays that pass directly through the light splitter 3 are focused onto one of two image scanners which may be called the azimuth scanner 4. This azimuth scanner lies in a plane normal to the tracking axis of the telescope 1. The azimuth scanner is comprised of a small, sector-shaped, mostly opaque mask 5 with a exible rod 6 attached to its inner radial side. The rod 6 is horizontally disposed and is rigidly attached to move with the body of the telescope 1. At an intermediate point along the rod 6 an electromagnet 7 is connected to vibrate the mask at a reference frequency in reference frequency line 8, in a plane parallel to the image focal plane of the telescope.

The line cut in the mask 5 by a vertical plane through the tracking axis of the telescope is called the control line 9 of the scanner. Windows 10 and 11 are cut in the opaque mask 5 to the right and left of the control line 9, respectively, and below and above the line, respectively, formed by the elongation of the vibrating rod 6 axis. The point where the windows touch is called the control point 12. Directly behind the mask, and also attached to move with the body of the telescope is positioned a photoelectric cell 15 which detects the position of the star image with respect to the control The output from the photocell 15 is amplified by azimuth amplifier 16 and led to an azimuth phase detector 17 which is supplied with the same reference frequency as the azimuth scanner 4 through reference frequency line connection 18. Hence, by means Well known to the art, energy initiated by photocell response is passed to an azimuth motor 20 to cause rotation in the proper direction through gear box 21 of a vertical azimuth shaft 22 supported by bearing 23 on platform 2 and connected to a yoke 24 supporting telescope 1. This mounting and connection enables the telescope 1 to be rotatable in azimuth in accordance with photocell response.

A means for indicating the azimuth angle is provided by a potentiometer resistance 30, only a portion of which is shown, having a contact arm 31 which is rigidly attached to vertical shaft 22 and adapted for movement therewith. The contact arm 31 is slidable over resistance 30, one extremity of which is connected by lead 32 to an ammeter 33 which may be calibrated for example as a function of telescope azimuth. The contact arm 31 is also connected to said ammeter by lead 34, there being a source of electric energy 35 in the last mentioned lead.

A second scanner which may be best described as an elevation scanner 4a is identical to the azimuth scanner 4 and is placed to receive the star image from the light reflected at substantially from the light splitter 3. The plane of the elevation scanner is vertical and its control line 9a is parallel to the tracking axis of the telescope. Elevation scanner rod 6a is attached to move with the telescope body and is disposed perpendicular to the longitudinal axis thereof. Rod 6a also vibrates at reference line frequency, the latter being led to elevation electromagnet 7a through elevation line connection 41. Elevation scanner mask 5a has windows 10a and 11a above and below the control line respectively, these windows again so arranged that the lower right corner of the upper window touches the upper left corner of the lower window. As in the azimuth circuit, the elevation photocell 15a is positioned directly behind the scanner 4a. The output of the photocell 15a is amplified by elevation amplifier 16a and led to an elevation phase detector 17a which senses the direction of the movement of the image over scanner 4a, as it is also connected to the same reference frequency line as line 41 through elevation reference frequency wire 42. The elevation power motor 20a thus rotates in response to the output of the elevation phase detector 17a to drive elevation shaft 44 through elevation gear box 21a. Elevation shaft 44 rests in fork bearing 46 and is rigidly attached to telescope body 1 to produce a change in telescope elevation when rotated. A support 47 for the elevation motor 20a and gear box 21a is rigidly connected to yoke 24 to rotate in coniunction with the telescope 1 about a vertical axis.

The elevation indicating arrangement for the telescope is obtained by means of a potentiometer resistance analogous to the one previously described. Contact arm 31a, rigidly attached to elevation shaft 44, is slidable over resistance 30a. Lead 32a connects one end of the resistance to elevation ammeter 33a. Lead 34a connects the contact arm to the ammeter, there being a source of electric energy 35a in the latter lead.

By placing the control line of azimuth scanner 4 at right angles to the control line elevation scanner 4a, angular deviations in both azimuth and elevation of the scanning axis of the telescope, with respect to the sight line of a star, are sensed by the scanner device. The point where the two control lines cross is positioned in the tracking axis and thus may be termed the control point. The system is stable only when the star image coincides with the control point, and when light from a star reaches the photocells by the image moving away from the control point in any direction', the motors act in the proper direction to move the telescope back to the angle where the image is again on the control point.

The particular tracking system just above described is no part of the present invention, being shown, described and claimed in the Tremble et al., application, Serial No. 81,226, filed March 14, 1949, and is shown and described herein merely as being illustrative of a particular device in which the optical system of the present invention can be utilized to advantage.

Two optical systems for use in telescope 1 are shown that provide an image field having both high image tracking accuracy and a wide angular field. These systems are diagrammatically shown in Figures 2 and 3.

Referring first to Figure 2, a cassegranian system is shown having a primary concave mirror 36 having a central aperture 36a herein. Forward of said primary mirror and positioned symmetrically in the tracking axis A of the system, is a mirror-lens 37 having its rear surface 37a partially silvered, to reflect light received from the primary mirror 36 and shaped to focus that light in an image field 39 behind the primary mirror, the light passing through the aperture 36a. This light path is identical with that of a conventional cassegranian reflector and provides a primary image in the image eld through an objective of relatively long focal length. This primary image, at or near the control point, gives the re quired accuracy of tracking control, but due to the relatively narrow angle of the field, the primary image will move out of the image field with relatively small angular displacements of the tracking axis from the sight line, and tracking control would then be lost.

However, mirror-lens 37 is provided with a front surface 40 contoured to cause the mirror-lens also to focus a star image in the image field 39. This can be done because the rear surface is silvered to provide partial light transmission. This secondary image is formed in a system having a relatively short focal length and therefore, this latter system has a relatively wide angle image field. Accordingly, after theV primary image has passed out of the image field due to a telescope deviation, the secondary image will remain in the image field until it also passes out of the field. However, the field of the secondary optical system can be made sufficiently wide that the secondary image will remain in the field during the maximum deviations that can normally be expected to occur during use of the system. The light path for the primary image is labelled P, and for the secondary image SEC.

A similar field expansion is obtained in the optical system shown in Figure 3. This figure also shows a cassegranian reliector of the same focal length as that in the system shown in Figure 2, and differs only in the respect that a secondary mirror 43, having a rear surface 45 that is fully reflective, is used instead of mirrorlens 37, and that a toroidal lens 50 is positioned between the secondary mirror 43 and the image field, preferably relatively close to the image field, as shown. The toroidal lens is composed of two portions, a central portion 48 that is linear and which has essentially no effect on the incoming light beam, and through which the primary image is formed in the primary field, and an outer toroidal portion 49 that is nonlinear. This outer portion maintains a secondary star image in the secondary field over wide angular deviations of the tracking axis from the sight line, the image being substantially radially stationary but being in the proper direction from the control point. The only image path shown is the secondary image path SEC. A light path providing a primary image would pass through only the central portion 48 of the toroidal lens 50, after the tracking axis is more nearly aligned with the sight line than it is at the condition illustrated.

In this case, when the telescope is moved over an angle which moves the primary image out of the primary field, the secondary image appears in the secondary field, the scanner senses the deviation and returns the image toward the control point. As the tracking axis approaches the sight line the primary image reappears in the primary field and the scanner continues to operate to center the primary image on the control point.

Thus, in both instances, high tracking accuracy is obtained at and near the control point, with a wide angle field available in which the secondary image appears after the primary image has left the central portion of the image field. In both instances, only the secondary image appears in the entire image field when the primary image disappears.

These two optical systems have slight differences. In the mirror-lens system, the primary and secondary images are not as bright, due to the light splitting at the partially reliective surface 3701. However, the primary field is the same width as the total image field, so that the scanner can work on the primary image over an angle as determined by the width of the entire field. In the toroidal lens system, the image is of full light value, but the primary field is small, being substantially less than the entire field width. The choice of the system to be utilized will, therefore, be influenced by the type of scanner utilized, the magnitudes of the stars to be tracked, etc., but in both systems the tracking accuracy will be the same for the same focal length of the cassegranian reflecting system at and near the control point.

While the present invention has been described as being ideally adapted for use in star tracking systems, it will be understood that the invention will also be useful in any device where a narrow angle field and a wide angle field are simultaneously required in the same optical system.

What is claimed is:

l. An optical system of the cassegranian type comprising an apertured primary mirror, a secondary mirror cooperating with said primary mirror to form a primary image of a point source on an image field behind said primary mirror, and lens means between said secondary mirror and said image field, said lens means having a linear central portion not affecting said primary image, and a non-linear outer portion to form a secondary image of said point source on said image field over a wider angle field of view than possible for said primary image.

2. An optical system of the cassegranian type comprising an apertured primary mirror, a secondary mirror cooperating with said primary mirror to form a primary image of a point source on an image field behind said primary image over a light path at and near the optical axis of said system, and annular non-linear lens means centered on said optical axis between said secondary mirror and said image field to form a secondary image of said point source of the same size as said primary image on an outer portion of said image field with a wider angle field of view than that for said primary image, said secondary image appearing in said field after said primary image has disappeared.

3. An optical system of the cassegranian type comprising an apertured primary mirror, a secondary mirror shaped to focus a primary image through said aperture on an image field back of said primary mirror, and a toroidal lens placed between said secondary mirror and said image field and shaped to focus a secondary image on said image field, said toroidal lens having a central portion of linear light-passing character coaxial with the optical axis of said system and an outer toroidal ,portionv surrounding said central portion, the focal plane of light from a distant source reflected from said primary mirror onto said secondary mirror and reflected from said secondary mirror through said central linear portion being at said image field to form said primary image', and the focal plane of light from the same source reflected from' said primary and secondary mirrors through said outer toroidal portion also being at said image field to form said secondary image, whereby said system has a high tracking accuracy of a point source of light when the (primary) image thereof is near the center of said image field and whereby a (secondary) image of the point source is retained on said image field over a wide angle of View.

4. In an optical system of the cassegranian type ha-ving an apertured primary mirror and a secondary mirror cooperating with said primary mirror to form a primary image on an image field behind said primary mirror, the improvement comprising a toroidal lens having a linear central portion and positioned on the optical axis of said system between said secondary mirror and said image field, said central portion not essentially affecting the position of said primary image, the outer portion of said toroidal lens intercepting the light path from a point source of light when said light path falls outside said central portion and defiecting the same back in the direction of said optical axis and focusing a point image of said source on said image field, to thereby provide a secondary image on said image field when the primary image would be beyond the extent of said field.

5. An optical system of the cassegranian type comprising an apertured primary mirror, a secondary mirror, an image field back of said primary mirror, and nonlinear lens means placed between said secondary mirror and said image field, said lens means having a linear central portion extending circularly around the optical axis of said system and not essentially affecting light rays passing therethrough from said secondary mirror to form a primary image on a central portion of said image field, and a light-converging outer portion of said lens means maintaining a substantially radially immovable secondary image on an outer portion of said image field in the presence of radially moving light rays passing therethrough from said secondary mirror.

References Cited in the file of this patent UNITED STATES PATENTS 729,848 Common June 2, 1903 2,198,014 Ott Apr. 23, 1940 2,326,552 Morse Aug. 10, 1943 '2,336,379 Warmisham Dec. 7, 1943 2,409,186 Bonwers Oct. 15, 1946 2,444,933 Jasperson July 13, 1948 2,513,367 Scott July 4, 1950 2,520,636 Grey Aug. 29, 1950 2,661,658 Dyson Dec. 8, 1953 OTHER REFERENCES Article of Albert G. Ingalls, published in Scientific American, December 1947, pages 283, 285 and 286. 

